Shopping.com reported record holiday activity, registering 54 percent growth in year-over-year traffic. Patrick Byrne, chief executive officer of Overstock.com, said the site had the biggest day in its history with about $10 million in sales, a 10 to 15 percent gain compared with last year. An average day for Overstock.com is $2 million in sales, he said.

Monday was also the highest-volume day at Shopstyle.com, which launched in February, and smaller sites reported heavy volume.

Signifying the importance of the day to online retailers, companies such as eBay, PayPal.com and Shopping.com have taken to calling it “Green Monday.”

Shopping.com, a comparison shopping Web site that works with more than 6,000 retailers, said Cyber Monday remains an important milestone because it marks the beginning of the online holiday shopping season, but activity peaks on the second Monday in December.

According to data from PayPal, the second Monday in December has been the busiest day for online holiday sales since 2001, and the volume continues to grow, including a 33 percent increase in payment volume last year compared with 2005.

Ugg boots are the most popular shoe of the season at more than one site, including nordstrom.com and Shopping.com. Nordstrom.com also reported that cashmere, coats, designer handbags, watches, premium denim, sweaters and fragrance were top-selling categories. Meanwhile, hot items on eBay.com are Juicy Couture by Juicy Couture fragrance, silver cuff bracelets, Burberry headbands and Anya Hindmarch’s I’m Not a Plastic Bag bag, which continues to be one of the most searched-for handbags on the site, a spokeswoman said.

Amazon.com is seeing high demand for diamond jewelry, from $49 for a one-tenth-carat diamond necklace to $2,199.99 for 1-carat platinum diamond stud earrings. Dresses, designer handbags and boots have been “huge” at Shopstyle.com, said Samantha Durbin, editor of FabSugar.com. “Popular designers are Christian Louboutin, Marc by Marc Jacobs, BCBG and Juicy Couture.”

“Our cart sizes are running way higher, they’re over $250,” said Andy Moss, ceo of Shopstyle.com. “We continue to see that trend increase as we get closer to holiday season. Post-season it’s all going to be about deals. We launched a new deals section, which will switch to sales in January.”

Apparel is strong at Overstock.com, which buys overruns from manufacturers, picked up fall goods when department stores cut back on reorders in October.

Alison Loehnis, vice president of sales and marketing at Net-a-porter.com, said: “On Monday, we saw an incremental lift of around 20 percent. People are clearly buying for the party season. The evidence is that cocktail dresses were our biggest seller.”